P-ohmic contact structure and photodetector using the same

ABSTRACT

A photodetector includes an UV transparent n-type structure, an UV transparent p-type structure, and a photon absorbing region sandwiched between the n-type structure and the p-type structure; a p-contact layer formed on the p-type structure; and a p-ohmic contact of a thickness in the range of 0.2-100 nm formed on the p-contact layer, wherein the p-ohmic contact comprises one or more layer of metal oxide.

FIELD OF THE INVENTION

The present disclosure relates in general to semiconductor lightemitting and/or detecting technology and, more particularly, to ap-ohmic contact structure and a light-emitting diode or a photodetectorusing the same.

DESCRIPTION OF THE RELATED ART

Nitride compound semiconductors such as InN, GaN, AlN, and their ternaryand quaternary alloys depending on alloy composition enable ultraviolet(UV) emissions ranging from 410 nm approximately to 200 nm. Theseinclude UVA (400-315 nm), UVB (315-280 nm), and part of UVC (280-200 nm)emissions. UVA emissions are leading to revolutions in curing industry,and UVB and UVC emissions owing to their germicidal effect are lookingforward to general adoption in food, water, and surface disinfectionbusinesses. Compared to the traditional UV light sources, such asmercury lamps, UV light emitters made of nitride compounds offerintrinsic merits. In general, nitride UV emitters are robust, compact,spectrum adjustable, and environmentally friendly. They offer high UVlight intensity and dosage, facilitating an idealdisinfection/sterilization treatment for water, air, food and objectsurface. Further, the light output of nitride UV light emitters can bemodulated at high frequencies up to a few hundreds of mega-hertz,promising them to be innovative light sources for Internet of Things,covert communications and bio-chemical detections.

A UV light-emitting diode (LED) comprises an n-type AlGaN structure, ap-type AlGaN structure, and a light-emitting structure commonly made ofAlGaN multiple-quantum-well (MQW) sandwiched in-between the n-type andp-type AlGaN structures. An AlGaN structure can be made of an AlGaNlayer or many AlGaN layers joint forces to deliver a better function,such as to improve material quality, conductivity and/or carrierconfinement. The deep level nature of acceptors in AlGaN materials makesp-type AlGaN structure highly resistive and not suitable for p-typeohmic contact formation, hence a thick (usually more than 100 nm-thick)p-type GaN layer is conventionally formed on top of the p-AlGaNstructure to serve as a p-contact layer for UV LEDs. The p-contact metalscheme for conventional UV LEDs includes a nickel (Ni) layer depositedon the p-type GaN layer and a following gold (Au) cap layer. As GaN hasa UV transmission cutoff edge at 365 nm, the thick p-GaN layer virtuallyabsorbs all UVB and UVC photons. Leftover photons if any, will furtherbe absorbed by Ni/Au p-contact. As such, the conventional UV LEDs are ofpoor efficiency, usually of less than 5% electrical-optical powerconversion efficiency (PCE).

As disclosed in U.S. Pat. No. 10,276,746, a two-dimensional hole gas(2DHG) can be formed and confined in the surface of an engineered AlGaNlayer utilizing the huge polarization fields of AlGaN materials. Thisengineered AlGaN layer can serve as a hole supplier and p-contact layer,enabling a UV transparent UV LED epitaxial structure. UV reflectivep-contact metal schemes are desired to maximize the light extraction andpower conversion efficiencies provided by this UV transparent UV LEDepitaxial structure.

SUMMARY

One aspect of the present disclosure provides a photodetector including:

an n-type structure, an UV transparent p-type structure, and a photonabsorbing region sandwiched between the n-type structure and the UVtransparent p-type structure;

an UV transparent p-contact layer formed on the UV transparent p-typestructure; and

a p-ohmic contact of a thickness in the range of 0.2-100 nm formed onthe p-contact layer, wherein the p-ohmic contact comprises one or morelayer of metal oxide.

Another aspect of the present disclosure provides another photodetectorincluding:

an n-type structure, an UV transparent p-type structure, and a photonabsorbing region sandwiched between the n-type structure and the UVtransparent p-type structure;

an UV transparent p-contact layer formed on the UV transparent p-typestructure; and

a p-ohmic contact on the UV transparent p-contact layer, wherein thep-ohmic contact is formed by sequentially depositing one or more layersof metal on the UV transparent p-contact layer and oxidizing the one ormore layers of metal.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and constitute a part of thisapplication, illustrate embodiments of the invention and together withthe description serve to explain the principle of the invention. Likereference numbers in the figures refer to like elements throughout, anda layer can refer to a group of layers associated with the samefunction.

FIG. 1 illustrates a UV LED epitaxial layered structure according to anembodiment of the present invention.

FIG. 2 illustrates a UV LED epitaxial layered structure according toanother embodiment of the present invention.

FIG. 3A plots current-voltage (IV) curves of two UVC LED chips with andwithout metal oxide p-ohmic contact according to an embodiment of thepresent invention.

FIG. 3B plots emission power and wavelength of two UVC LED chips withand without metal oxide p-ohmic contact according to an embodiment ofthe present invention.

FIG. 4A plots current-voltage (IV) curves of two UVC LED chips with andwithout metal oxide p-ohmic contact according to an embodiment of thepresent invention.

FIG. 4B plots emission power and wavelength of UVC LED chips with andwithout metal oxide p-ohmic contact according to an embodiment of thepresent invention.

FIG. 5A plots current-voltage (IV) curve of a UVC LED chip according toan embodiment of the present invention.

FIG. 5B plots emission power and wavelength of a UVC LED chip accordingto an embodiment of the present invention.

FIG. 6 plots SIMS elements depth profiles of the metal oxide p-ohmiccontact of the LED used in FIGS. 3A and 3B.

FIG. 7 plots SIMS elements depth profiles of the metal oxide p-ohmiccontact of the LED used in FIGS. 4A and 4B.

FIG. 8 plots reflectance curve of a 65-nm-thick Al reflector.

FIG. 9 plots the normalized reflectance curves (to the 65-nm-thick Alreflector reflectance curve shown in FIG. 8) of various reflectorsaccording to one aspect of the present invention.

FIG. 10 illustrates a UV photodetector epitaxial layer structureaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Throughout the specification, the term group III nitride in generalrefers to metal nitride with cations selecting from group IIIA of theperiodic table of the elements. That is to say, III-nitride includesAlN, GaN, InN and their ternary (AlGaN, InGaN, InAlN) and quaternary(AlInGaN) alloys. In this specification, a quaternary can be reduced toa ternary for simplicity if one of the group III elements issignificantly small so that its existence does not affect the intendedfunction of a layer made of such material. For example, if theIn-composition in a quaternary AlInGaN is significantly small, smallerthan 1%, then this AlInGaN quaternary can be shown as ternary AlGaN forsimplicity. Using the same logic, a ternary can be reduced to a binaryfor simplicity if one of the group III elements is significantly small.For example, if the In-composition in a ternary InGaN is significantlysmall, smaller than 1%, then this InGaN ternary can be shown as binaryGaN for simplicity. Group III nitride may also include small amount oftransition metal nitride such as TiN, ZrN, HfN with molar fraction notlarger than 10%. For example, III-nitride or nitride may includeAl_(x)In_(y)Ga_(z)Ti_((1-x-y-z))N, Al_(x)In_(y)Ga_(z)Zr_((1-x-y-z))N,Al_(x)In_(y)Ga_(z)Hf_((1-x-y-z))N, with (1−x−y−z)≤10%.

As well known, light-emitting devices such as light-emitting diodes(LEDs) and laser diodes, commonly adopt a laminate structure containinga quantum well active region, an n-type group III nitride structure forinjecting electrons into the active region, and a p-type group IIInitride structure on the other side of the active region for injectingholes into the active region.

Illustrated in FIG. 1 is a cross-sectional schematic view of a UV LEDstructure according to an embodiment of the present invention. Thestructure starts with a UV transparent substrate 10. Substrate 10 can beselected from sapphire, AlN, SiC, and the like. Formed over substrate 10is a template 20, which can be made of a thick AlN layer, for example,with a thickness of 0.3-4.0 μm. Even though not shown in FIG. 1, astrain management structure such as an Al-composition grading AlGaNlayer or sets of AlN/AlGaN superlattices can be formed over template 20.Formed over template 20 is a thick n-AlGaN structure 30 for electronsupply and n-type ohmic contact formation. Structure 30 may include athick (2.0-5.0 μm such as 3.0 μm, n=2.0×10¹⁸-5.0×10¹⁸ cm⁻³) n-typeN—AlGaN layer 31 for current spreading, a heavily n-type doped (0.2-0.8μm such as 0.60 μm, n=8×10¹⁸-2×10¹⁹ cm⁻³) NtAlGaN layer 33 for MQWactive-region polarization field screening, and a lightly doped N⁻—AlGaNlayer 35 (0.1-0.5 μm such as 0.3 μm, n=2.5×10¹⁷-2×10¹⁸ cm⁻³) to reducecurrent crowding and prepare uniform current injection into thefollowing Al_(b)Ga_(1-b)N/Al_(w)Ga_(1-w)N MQW active-region 40. MQW 40is made of alternatingly stacked n-Al_(b)Ga_(1-b)N barrier andAl_(w)Ga_(1-w)N well for a few times, for example, for 3-8 times. Thebarrier thickness is in the range of 8-16 nm, and the well thickness is1.2-5.0 nm. The total thickness of MQW 40 is usually less than 200 nm,for example, being 75 nm, 100 nm, or 150 nm. The n-Al_(b)Ga_(1-b)Nbarrier and Al_(w)Ga_(1-w)N well may have an Al-composition in the rangeof 0.3-1.0, and 0.0-0.85, respectively, and the Al-compositiondifference of the barrier and well is at least 0.15, or so to ensure abarrier-well bandgap width difference (ΔE_(g)) at least 400 meV tosecure quantum confinement effect. Following MQW 40 is a p-type AlGaNstructure 50. Structure 50 can be a p-AlGaN layer of uniform or varyingAl-composition, or a p-AlGaN superlattice structure, or a p-AlGaN MQWstructure, or a p-AlGaN multilayer structure serving as hole injectingand electron blocking layer. Structure 50 has enough Al-composition andmodulation to allow for sufficient electron blocking and hole injectionefficiencies. Further, structure 50 is also efficient in spreading holecurrent laterally. Formed on top of structure 50 is a hole supplier andp-contact layer 60, which can be engineered according to U.S. Pat. No.10,276,746, the content of which is herewith incorporated by referencein its entirety, to have surface hole gas accumulation for p-type ohmiccontact formation. Briefly, p-contact layer 60 is a thin (0.6-10 nm),strained, and heavily acceptor-doped nitride layer (e.g. Mg-doped, to aconcentration about 10²⁰ cm⁻³ or more). For UVB/UVC LEDs (emissions from200 nm-315 nm), p-contact layer 60 prefers to be a Mg-doped AlGaN layerwith Al-composition larger than 0.7, or with Al-composition to be from0.7 to 1.0. If MQW active-region 40 emits longer wavelength emissions,for example UVA emissions (315 nm-400 nm), or visible emissions,p-contact layer 60 can have less Al-composition.

The surface high-density 2DHG of p-contact layer 60 can form good ohmiccontact to many metals, not only to high-work-function metals likeNickel (Ni), Tungsten (W), Molybdenum (Mo) Palladium (Pd), Platinum(Pt), Iridium (Ir), Osmium (Os), Rhodium (Rh) and Gold (Au), but also tosome low-work-function metals like UV reflective metal Aluminum (Al) andvisible light reflective Silver (Ag) and Indium (In). In thisspecification, high work-function means that the work function is largerthan 5.0 eV, and low work-function means that the work function is lessthan 5.0 eV.

Optionally, referring to FIG. 2, hole supplier and p-contact layer 60can have an assistant p-contact layer (APC) 61 formed thereon. Assistantp-contact layer 61 is thin, 0.2-2 nm-thick, made of p-type AlGaN withsmall (for example, Al-composition smaller than 0.4) or even vanishingAl-composition, to provide protection to p-contact layer 60. Assistantp-contact layer 61 also holds 2DHG to the vicinity of surface.

Referring to FIGS. 1 and 2, a mesa is etched out to expose N—AlGaNstructure 30, for deposition of n-ohmic contact 81, which can be made ofthin metal layer stacks such as titanium/aluminum/titanium/gold(Ti/Al/Ti/Au) with respective layer thickness of30-40/70-80/10-20/80-100 nm, for example 35/75/15/90 nm, or V/Al/V/Ag,V/Al/V/Au, and V/Al/Ti/Au, of respective thicknesses such as20/60/20/100 nm. As seen from FIGS. 1 and 2, n-ohmic contact 81 ispreferred to be formed on the heavily n-type doped NtAlGaN layer 33.Overlying n-ohmic contact 81 is n-contact pad 89 made of a thick (2-10μm) gold or gold tin layer.

Referring to FIGS. 1 and 2, respectively formed on hole supplier andp-contact layer 60 and/or assistant p-contact layer 61 is a p-ohmiccontact 71. According to an embodiment of the present invention, p-ohmiccontact 71 is a metallic contact containing element oxygen (O). Thismeans that p-ohmic contact 71 contains metal and oxygen elements,however, it does not need to be perfect stoichiometric metal oxides.According to an embodiment of the present invention, metallicoxygen-containing p-ohmic contact 71 formed over hole supplier andp-contact layer 60 and/or assistant p-contact layer 61 provides at leastthree merits. The first one is to improve contact stability. Metal atomsare susceptible to electric field-driven diffusion. Using metallicoxygen-containing p-ohmic contact can greatly reduce metal atomsfield-driven diffusion, hence improve contact stability. The second oneis that the formation of metallic oxygen-containing p-ohmic contact 71according to an embodiment of the present invention enables 0 and Mgco-doping in p-type AlGaN structure 50. Proper O and Mg co-doping canreduce Mg acceptors' activation energy, presumably through the getteringeffect of the deleterious hydrogen (H) atoms abundant in Mg-doped AlGaNlayers. The third merit is that metallic oxygen-containing p-ohmiccontact can reduce light absorption loss as compared to metal p-ohmiccontact.

In the following content, for simplicity of description, p-ohmic contact71 is said to be made of metal oxide, where the metal cations can beselected from nickel (Ni), indium (In), tin (Sn), copper (Cu), aluminum(Al), gallium (Ga), chromium (Cr), molybdenum (Mo), strontium (Sr),scandium (Sc), Yttrium (Y), zinc (Zn), rhodium (Rh), iridium (Ir),cobalt (Co), osmium (Os), palladium (Pd), platinum (Pt), and ruthenium(Ru).

P-ohmic contact 71 can be formed via thermal or chemical deposition oneor more layers of metal oxide on p-contact layer 60 or assistantp-contact layer 61. The metal oxide can be a binary oxide, such asNiO_(x) (NiO), InO_(x) (In₂O₃), SnO_(x) (SnO, SnO₂), RhO_(x) (Rh₂O₃),MoO_(x), and IrO_(x), et al. These binary oxides do not have to bestoichiometric. According to embodiments of the present invention, themeasurable oxygen level in the metal oxides can be from some dopinglevel up to stoichiometric level, e.g., from 10¹⁹ cm⁻³ up tostoichiometric level, or from 10²⁰ cm⁻³ up to stoichiometric level, orfrom 10²¹ cm⁻³ up to stoichiometric level. The metal oxide can be aternary oxide, such as CuMO_(x) (such as x=2), where metal M can beselected from Al, Ga, In, Cr, Sr, Sc, and Y, or, be spinel type oxide,ZnM₂O_(x) (such as x=4), where M=Rh, Ir, or Co. Optionally, the metaloxides possess p-type conductivity.

P-ohmic contact 71 may include one layer of metal oxide selected fromthe above discussed metal oxide. The thickness of the metal oxide layercan be in the range of 0.2-100 nm, such as 1 to 20 nm, or 2 to 10 nm.

P-ohmic contact 71 may include two layers of metal oxide, such as alayer of NiO_(x) and a layer of InO_(y), a layer of InO_(x) and a layerof RhO_(y), a layer of NiO_(x) and a layer of MoO_(y), a layer ofRhO_(x) and a layer of MoO_(y), a layer of NiO_(x) and a layer ofRhO_(y), et al. For example, p-ohmic contact 71 may include a firstmetal oxide layer and a second metal oxide layer formed on the firstmetal oxide layer selected from the following pairs:

a first layer NiO_(x) and a second layer MoO_(y), InO_(y) or RhO_(z); afirst layer MoO_(x) and a second layer NiO_(y), InO_(y) or RhO_(y); anda first layer InO_(x) and a second layer NiO_(y), MoO_(y), or RhO_(y),et al. The thickness of the first metal oxide layer can be 0.1%-5% ofthe total thickness of the first and second metal oxide layers. Again,the measurable oxygen level in these metal oxides can be from somedoping level up to stoichiometric level, e.g., from 10¹⁹ cm⁻³ up tostoichiometric level.

P-ohmic contact 71 may include three layers of metal oxide such as, inthe sequence of first, second and third metal oxide layer with the firstlayer facing p-contact layer 60 or assistant p-contact layer 61, a layerof InO_(x), a layer of NiO_(y) and a layer of AuO_(z); a layer ofNiO_(x), a layer of RhO_(y) and a layer of MoO_(z); a layer of NiO_(x),a layer of MoO_(y) and a layer of RhO_(z); et al. The thickness of thefirst metal oxide layer can be 0.1%-5% of the total thickness of thethree metal oxide layers, and the ratio of the thickness of the secondmetal oxide layer to the thickness of the third metal oxide layer can bein the range of 1-10.

P-ohmic contact 71 can also be formed via oxidization of metal layer(s)formed on p-contact layer 60 or assistant p-contact layer 61, such asvia oxygen plasma treatment of the respective metal layer(s), or thermalannealing of the respective metal layer(s) in oxygen-containing ambient.Oxygen-containing ambient as used in this disclosure means that theambient or atmosphere contains oxygen, or water vapor, or oxygen/watervapor mixed with other suitable gases, such as nitrogen, oxygen, watervapor, argon, et al.

In some embodiments, a p-ohmic contact 71 is formed by sequentiallydepositing one or more layers of metal on the p-contact layer 60 orassistant p-contact layer 61 and oxidizing the one or more layers ofmetal such as via thermal annealing or oxygen plasma treatment. Themetal can be selected from nickel (Ni), indium (In), tin (Sn), copper(Cu), aluminum (Al), gallium (Ga), chromium (Cr), molybdenum (Mo),strontium (Sr), scandium (Sc), Yttrium (Y), zinc (Zn), rhodium (Rh),iridium (Ir), cobalt (Co), osmium (Os), palladium (Pd), platinum (Pt),and ruthenium (Ru), a mixture of two, three, or four of these metals.

In some embodiments, p-ohmic contact 71 is formed by sequentiallydepositing two layers of metal on p-contact layer 60 or assistantp-contact layer 61 and oxidizing the two layers of metal, the two layersof metal include a first layer formed on p-contact layer 60 or assistantp-contact layer 61 and a second layer formed on the first layer and areselected from:

a Ni layer on p-contact layer 60 or assistant p-contact layer 61 and aMo layer on the Ni layer, or an In layer on the Ni layer, or a Rh layeron the Ni layer; a Mo layer on p-contact layer 60 or assistant p-contactlayer 61 and a Ni layer on the Mo layer, or an In layer on the Mo layer,or a Rh layer on the Mo layer; an In layer on p-contact layer 60 orassistant p-contact layer 61 and a Ni layer on the In layer, or a Molayer on the In, or a Rh layer on the In layer; a Rh layer on p-contactlayer 60 or assistant p-contact layer 61 and a Mo layer on the Rh layer,or a Ni layer on the Rh, or a Pd layer on the Rh layer. The thickness ofthe first layer can be in the range of 0.2-4 nm such as 0.5-2 nm, andthe thickness of the second layer can be in the range of 2-100 nm, suchas 2-20 nm or 5-10 nm.

In some embodiments, the p-ohmic contact 71 is formed by sequentiallydepositing three layers of metal on p-contact layer 60 or assistantp-contact layer 61 and oxidizing the three layers of metal, the threelayers of metal include a first layer formed on p-contact layer 60 orassistant p-contact layer 61, a second layer formed on the first layerand a third layer formed on the second layer, and are selected from Ni,Rh, Mo, Pd, Ir, Os and Ru, respectively. The thickness of the firstlayer, the second layer and the third layer are in the range of 0.2-100nm, 1-100 nm, and 1-100 nm, or optionally in the range of 0.2-2 nm, 1-20nm, and 1-20 nm, respectively.

In some embodiments, during the process of annealing the one or morelayers of metal in oxygen-containing ambient, oxygen also penetratesinto p-contact layer 60 and p-type structure 50.

In embodiments where contact 71 is made via oxidizing a single metal,the single metal layer such as Ni, In, Sn, Mo, Rh, or Ir, et al can bedeposited via e-beam vapor deposition with calibrated deposition rate.In embodiments where contact 71 is made via oxidizing two or moremetals, respective metal can be deposited in sequence with respectivelayer thickness, or different metals can be deposited at the same timewith different deposition rate to obtain targeted composition or mixtureratio of different metals.

When the metal oxide of p-ohmic contact 71 is formed by oxidizingcorresponding metal layer(s) such as via thermal annealing or oxygenplasma treatment, the metal oxide layer(s) resulted from oxidizing themetal layer(s) may not have a clear boundary between layers due tointer-diffusion of metal atoms. For example, when p-ohmic contact 71 isformed by depositing two or more layers of metals and annealing the twoor more metal layers in oxygen-containing ambient, during the annealingprocess, metals in the two or more layers will diffuse into each otheracross the boundary between layers and may even diffuse through theentire thickness of p-ohmic contact 71, while oxygen penetrates into themetal layers. In such cases, the boundary between initial metal layersbecomes blurry, and the concentrations of each metal as well as oxygenmay vary along the thickness of p-ohmic contact 71. In some embodiments,each of the metals and oxygen diffuses through the entire thickness ofp-ohmic contact 71 and the concentrations of each metal as well asoxygen vary along the entire thickness of p-ohmic contact 71.

In some embodiments, p-ohmic contact 71 includes two or more types ofmetals diffusing into each other's phase and diffusing through theentire thickness of p-ohmic contact 71, and oxygen penetrates intop-ohmic contact 71 through its entire thickness (See FIGS. 6 and 7 forexample). The measurable oxygen level in p-ohmic contact 71 can be fromsome doping level up to stoichiometric level, e.g., from 10¹⁹ cm⁻³ up tostoichiometric level, or from 10²⁰ cm⁻³ up to stoichiometric level, orfrom 10²¹ cm⁻³ up to stoichiometric level. In the p-ohmic contact 71,the metals can be selected from nickel (Ni), indium (In), tin (Sn),copper (Cu), aluminum (Al), gallium (Ga), chromium (Cr), molybdenum(Mo), strontium (Sr), scandium (Sc), Yttrium (Y), zinc (Zn), rhodium(Rh), iridium (Ir), cobalt (Co), osmium (Os), palladium (Pd), platinum(Pt), and ruthenium (Ru).

In cases where p-ohmic contact 71 contains two types of metals, the twotypes of metals can be selected from the following pairs: first metal Niand second metal Mo, In, or Rh; first metal Mo and second metal Ni, In,or Rh; first metal In and second metal Ni, Mo, or Rh. The molar fractionof the first metal is 0.1%-5% of the total moles of the first metal andthe second metal.

In cases where p-ohmic contact contains three types of metals, the threetypes of metals include a first metal, a second metal and a third metal,and can be selected from Ni, Rh, Mo, Pd, Ir, Os and Ru. A molar fractionof the first metal, the second metal and the third metal are in therange of 0.1%-5%, 80%-90%, and 5%-19.9%, respectively. For example, thefirst metal can be Ni, and the second and third metals can be Rh and Mo,respectively, or, the first metal can be Ni, and the second and thirdmetals can be Rh and Pd, respectively, et al.

In the above embodiments, oxygen may penetrate into p-contact layer 60and p-type structure 50. The oxygen level in p-contact layer 60 andp-structure 50 can be in the range of 10²⁰-10²¹ cm⁻³ and 10¹⁹-4.0×10²⁰cm⁻³, respectively.

According to an embodiment of the present invention, the thermalannealing temperature or oxidation temperature depending on annealingambient and metals deposited on p-contact layer 60 or assistantp-contact layer 61 is usually higher than 450° C., for example, in therange of 450-850° C., or 500-750° C. And the thermal annealing time oroxidation time depending on metal, metal film thickness, annealingtemperature and ambient is usually more than 1 minute, for example, inthe range of 1-20 minutes, or 2-10 minutes.

The oxidation process can be confirmed via analytic techniques such asSecondary Ion Mass Spectrometry (SIMS), Energy Dispersive X-Ray (EDX)and X-ray photoelectron spectroscopy (XPS). SIMS is a technique used toanalyze elements of solid surfaces and thin films by sputtering thesurface of the specimen with a focused primary ion beam and collectingand analyzing ejected secondary ions. SIMS has high detectionsensitivity, can detect impurity levels as low as 10¹⁵ cm⁻³, but is notsuitable for composition determination. EDX analysis usually is combinedin Transmission Electron Microcopy (TEM) or Scanning Electron Microscopy(SEM), where an electron beam is used to hit targeted atoms and knockoff an electron from the atom inner shell, leaving a positively chargedmetastable atom. An outer shell electron will make quantum transition tofill the inner shell vacancy, releasing potential energy in the form ofX-ray. The energy of this X-ray is unique to the specific element andtransition so that EDX is widely applied to identify elements. EDXtherefore can be used to check the oxidation process via looking intooxygen signature EDX peaks. EDX has relative low detection sensitivitybut can be used to determine composition of alloys. It can be used todetect element with composition more than 1%. XPS on the other hand isbased on photoelectric effect, using X-ray to knock off electrons fromtargeted atoms. The kinetic energy of the photoelectric electrons can beused to identify elements and their chemical states. For example, oxygensignature XPS peaks (O 1s) can be used to quantify metal oxidationprocess. Further, per oxidation, the inner shell peaks of metals (suchas 3d peaks) can be shifted to higher energy as oxidation increasestheir binding energies. XPS is very sensitive to film surface chemicalchanges.

Formed on p-ohmic contact 71 is a metal reflector layer 73, which can beselected from metals Al, Pd, Pt, Os, Rh, Ir, In, Mo, and tungsten (W).Metal reflector layer 73 is preferably thicker than 10 nm, preferably 50nm-thick, or 100 nm-thick. Optionally, metal reflector layer 73 isUV-reflective, for example, UVC-reflective, to maximize light extractionefficiency. In one embodiment, p-ohmic contact 71 is of a thickness0.2-100 nm and metal reflector layer 73 is an Al layer of a thickness90-110 nm, for example 100 nm. In another embodiment, p-ohmic contact 71is of a thickness 0.2-2.0 nm and metal reflector layer 73 is a 75-nm Rdlayer. In another embodiment, p-ohmic contact 71 is metal oxide ofthickness 0.2-10.0 nm and metal reflector layer 73 is a Rd layer of athickness 60-70 nm, for example 65 nm. In another embodiment, p-ohmiccontact 71 is of a thickness 0.2-15.0 nm and metal reflector layer 73 isan Al layer of a thickness 70-80 nm, for example 75 nm. In still anotherembodiment, p-ohmic contact 71 is of a thickness 0.2-6.0 nm and metalreflector layer 73 is a Mo layer of a thickness 65-75 nm, for example 70nm. In yet another embodiment, p-ohmic contact 71 is of a thickness0.2-3.0 nm and metal reflector layer 73 is a Pd layer of a thickness75-85 nm, for example 80 nm.

Formed on metal reflector layer 73 is a thick metal layer serving asp-contact pad 79, which can be made of a 2-8 μm gold layer or gold tinlayer.

EXAMPLES

Three UVC LED wafers (#306, #394, #415) in the present disclosure weremade using Metal Organic Chemical Vapor Deposition (MOCVD) according tothe epitaxial structures shown in FIGS. 1 and 2. C-plane sapphire wasused as substrate 10, with a 3.5 μm-thick AlN serving as AlN template20. The N—AlGaN layer 31 is made of 2.5 μm-thick Al_(0.58)Ga_(0.42)Ndoped with Si to a level of 4.5×10¹⁸ cm⁻³, and the NtAlGaN layer 33 andN⁻—AlGaN layer 35 of the same composition to the N—AlGaN layer 31 are450 nm and 200 nm thick and doped with Si to 8.5×10¹⁸ cm⁻³ and 3.5×10¹⁷cm⁻³, respectively. The MQW 40 used was a five-pairmultiple-quantum-well, with the barrier thickness and Al-composition tobe 11.0 nm and 55%, and the well thickness and Al-composition to be 1.8nm and 35%, respectively. Further the barriers were doped with Si at3.0×10¹⁸ cm⁻³, and the wells were undoped. The p-AlGaN structure 50 usedwas an 8-pair AlGaN/AlGaN multiple-quantum-well structure, with barrierthickness and Al-composition to be 8.0 nm and 68%, respectively, andwell thickness and Al-composition to be 1.6 nm and 55%, respectively.Hole supplier and p-contact layer 60 was formed directly on the lastquantum well of p-AlGaN structure 50.

The three wafers, namely, wafers #306, #394, #415 were different interms of hole supplier and p-contact layer. Wafer #306 had a holesupplier and p-contact layer 60 made of 1.2 nm Mg-doped AlN layer (referto FIG. 1), whereas wafers #394 and #415 had p-contact layer 60 made ofa Mg-doped 1.5 nm Al_(0.82)Ga_(0.18)N with an additional assistantp-contact layer (APC) 61, which was a 0.8 nm-thick Mg-doped GaN layer(refer to FIG. 2).

The three wafers were fabricated into UVC LED chips using standardsemiconductor lithography, etch, and metallization processes.Ti/Al/Ti/Au multi layers with respective layer thickness of 35/75/15/90nm were used as n-ohmic contact (81). Wafers #306 and #415 were used tocompare Rh/Ni and RhO_(x)/NiO_(y) p-ohmic contacts (71), and wafer #394was used to compare Au/Ni/In and AuO_(x)/NiO_(y)/InO_(z) p-ohmiccontacts (71). For this, a 0.5 nm Ni layer then a 98.5 nm Rh layer(Ni/Rh thickness ratio ˜0.51%) were deposited on p-contact layer 60 ofwafer #306 and on assistant p-contact layer 61 of wafer #415, and a 50nm/50 nm/50 nm In/Ni/Au (In deposition first) metal stack was depositedon assistant p-contact layer 61 of wafer #394. After p-metal deposition,metal p-ohmic contacts and metal oxide p-ohmic contacts were formed viathermal annealing of the wafers under pure nitrogen ambient and underoxygen-containing ambient, respectively.

Shown in FIG. 3A are current-voltage (IV) curves of representative UVCLED chips made from wafer #415 with metal (Rh/Ni) and metal oxide(RhO_(x)/NiO_(y)) p-ohmic contacts, respectively. Their respectiveemission power and wavelength were compared In FIG. 3B. As seen, theforward voltage was reduced and emission power was increased for the UVCLED chip with metal oxide p-ohmic contact. The emission peak wavelengthwas at 266 nm and blue shifted to 262 nm when current increased from 20to 100 mA. The formation of metal oxide p-ohmic contact (71) viaannealing of Rd/Ni films under oxygen-containing ambient was confirmedby SIMS measurements. The results are shown in FIG. 6, where depthprofiles of elements O, Ni, and Rh are plotted for the metal oxidep-ohmic contact of the LED used in FIGS. 3A and 3B. As seen, uponthermal annealing, Ni diffused into Rh and mixed well with Rh, with twocomposition peaks, one in the proximity to the surface due to outdiffusion and surface segregation, another at the initial depositionplace which was interfaced with the LED p-type structure (assistantp-contact layer 61). Also evident is that the detected secondary ionintensity of Ni is relatively low because of the very thin metalthickness (nominal 0.5 nm). At least one of the functions of this verythin Ni layer is to enhance surface adhesion for the following thick Rhlayer. The oxygen depth profile followed the Ni's trend, and evenextended into the LED's p-type structure. This means that the formationof metal oxide p-ohmic contact 71 according to an embodiment of thepresent invention enables 0 and Mg co-doping in p-type AlGaN structure50. Proper O and Mg co-doping can reduce Mg acceptors' activation energyand improve p-type doping efficiency, presumably through the getteringeffect of the deleterious H atoms abundant in Mg-doped AlGaN layers. Assuch, seen from FIGS. 3A, 3B and 6, it is evident that RhO_(x)/NiO_(y)p-ohmic contact is formed and outperforms Rh/Ni metal p-ohmic contact.

Shown in FIG. 4A are IV curves of representative UVC LED chips made fromwafer #394 with Au/Ni/In and AuO_(x)/NiO_(y)/InO_(z) p-ohmic contacts,respectively. Their respective emission power and wavelength werecompared In FIG. 4B. As seen, the turn-on voltage was reduced andemission power was increased for the UVC LED chip withAuO_(x)/NiO_(y)/InO_(z) p-ohmic contact. The emission peak wavelengthwas at 262 nm and blue shifted to 258 nm when current increased from 20to 100 mA. The formation of metal oxide p-ohmic contact (71) viaannealing of Au/Ni/In films under oxygen-containing ambient wasconfirmed by SIMS measurements. Shown in FIG. 7 are SIMS elements depthprofiles of the metal oxide p-ohmic contact of the LED used in FIGS. 4Aand 4B. As seen, upon thermal annealing, metal interdiffusion wassignificant, so the mixed metal oxide thickness measured by SIMS becameabout 100 nm instead of 150 nm as deposited. In and Ni both out diffusedinto Au and mixed well with Au, each forming two concentration peaks onearound the surface and another at the initial film deposition place. Theoxygen depth profile followed the Ni's and In's trend, and extended intothe LED's p-type structure. As such, seen from FIGS. 4A, 4B and 7, it isevident that AuO_(x)/NiO_(y)/InO_(z) p-ohmic contact is formed andoutperforms Au/Ni/In metal p-ohmic contact.

Shown in FIG. 5A is IV curve of a representative UVC LED chip made fromwafer #306 with RhO_(x)/NiO_(y) p-ohmic contacts. Its emission power andwavelength were plotted In FIG. 5B. As seen, the forward voltage was7.0V at 100 mA, with chip-on-wafer optical power 21 mW. The emissionpeak wavelength was at 260 nm and blue shifted to 257 nm when currentincreased from 20 to 100 mA. Comparing FIGS. 3A, 3B, 5A and 5B indicatedthat getting rid of assistant p-contact layer 61 increased emissionpower without voltage penalty.

Metal oxide p-ohmic contact 71 according to an embodiment of the presentinvention may have certain UV absorption coefficient, therefore itsthickness may affect the effective reflectance of the combined reflectorof p-ohmic contact 71 and metal reflector layer 73. Shown in FIG. 8 is ameasured reflectance curve of an Al reflector, made of a 65 nm-thick Alfilm deposited on a double side polished sapphire substrate. As seen,this Al reflector has reflectance 90% for light in the wavelength regionof 240-300 nm. Shown in FIG. 9 are normalized reflectance (normalized tothe Al-reflector reflectance as shown in FIG. 8) curves of variouscombined reflectors according to another aspect of the presentinvention. These combined reflectors include a 65-nm-thick Al reflector,a 65-nm-thick Rh reflector, a 0.5/98.5 nm-thick NiO_(x)/RhO_(y)reflector, a 0.5/10 nm-thick NiO_(x)/RhO_(y) plus 65 nm Rh combinedreflector, and a 0.5/10 nm-thick NiO_(x)/RhO_(y) plus 65 nm Al combinedreflector. As seen, the 65-nm-thick Rh reflector has normalizedreflectance 60-66%, i.e., reflectance 54-59.4% for light in thewavelength region of 250-300 nm. The 0.5/10 nm-thick NiO_(x)/RhO_(y)plus 65 nm Rh combined reflector has very similar (yet negligiblyhigher) reflectance to the 65-nm-thick Rh reflector. Meanwhile, the0.5/10 nm-thick NiO_(x)/RhO_(y) plus 65 nm Al combined reflector hasvery similar reflectance to the 65-nm-thick Al reflector in thewavelengths above 240 nm, i.e., with ˜90% reflectance. The 0.5/98.5nm-thick NiO_(x)/RhO_(y) reflector has normalized reflectance28.4-30.6%, i.e., reflectance 25.6-27.5% for light in the wavelengthregion of 240-300 nm.

According to another aspect of the present invention, p-ohmic contact 71together with metal reflector layer 73 forms a combined reflector.Therefore, the thickness of metal oxide p-ohmic contact 71 can be from0.2 to 100 nm, such as from 1 to 20 nm, or from 2 to 10 nm.

The present disclosure has used UV LEDs as exemplary embodiments. It isnoted that these metal oxides p-ohmic contact can also be used for otheroptical devices, such as laser diodes and photodetectors. The majordifference of a photodetector and an LED lies in their active-regions.The active-region of an LED is commonly made of MQW for confiningelectrons and holes for enhanced radiative recombination rate, whereasthe active-region of a photodetector is a light (photon) absorbing thicksemiconductor layer, used to generate photon-induced electrons andholes, which are separated by a reversed bias to the PN junction togenerate photocurrent for photon detection.

Illustrated in FIG. 10 is a schematic layer structure of a photodetectorusing transparent hole supplier and p-contact layer 60 and metal oxidep-ohmic contact 71 according to an embodiment of the present invention.In the photodetector, substrate 10, template layer 20, n-AlGaN structure30, p-AlGaN structure 50, transparent hole supplier, p-contact layer 60and metal oxide p-ohmic contact 71, metal reflector layer 73, p-contactpad 79, n-ohmic contact 81, and n-contact pad 89 can be the same as ordifferent than those discussed above in connection with the LEDstructure.

As seen, the photodetector can be formed over a high-quality template orwindow layer 20, which in turn can be formed on a substrate 10. Forsolar blind applications, substrate 10 can be a sapphire or AlN wafer,and template layer 20 can be an AlN layer or AlGaN layer withAl-composition higher enough to assuring UV light transparency to thePhoton Absorbing Region (Phar) 40′ (photodetector's active-region),which can be made of intrinsic AlGaN with a thickness of 100-500 nm,such as 200-300 nm. The thickness of photon absorbing region 40′ isarranged so that enough photons are absorbed and photocurrent isgenerated. In practice, the Al-composition of window layer 20 can be atleast 20%, or 30%, or 50% more than that of the photon absorbing region40′. For example, the Al-composition of window layer 20 can be in therange of 0.6-1.0, while the Al-composition of photon absorbing region40′ is 0.46 for 280 nm and shorter wavelength detection. N—AlGaNstructure 30 can be a Si-doped AlGaN layer transparent to the targeteddetection wavelength with an Al-composition in the range of 0.5-0.7. Forsolar blind detection of 280 nm and shorter wavelengths, the targetedAl-composition of the intrinsic AlGaN material for photon absorbingregion 40′ is not less than 0.46 (here calculated by assuming GaN andAlN bandgap energy to be respectively 3.42 and 6.2 eV, and bowingparameter for AlGaN bandgap energy to be −1), for example, 0.46 to 1.0,or 0.47 to 0.55. Formed on photon absorbing region 40′ is a p-AlGaNstructure 50, which can be a p-type AlGaN layer with Al-compositionlarger than or equal to that of N—AlGaN structure 30. N—AlGaN structure30 and p-AlGaN structure 50 can also be the same or similar to theircounterparts found in a UV LED structure such as shown in FIGS. 1 and 2.Formed on p-AlGaN structure 50 is a hole supplier and p-contact layer60, which is followed by a metal oxide p-ohmic contact 71 and a UVreflective p-metal layer 73, capped with a thick metal p-contact pad 79.

The present disclosure has been described using exemplary embodiments.However, it is to be understood that the scope of the present inventionis not limited to the disclosed embodiments. On the contrary, it isintended to cover various modifications and similar arrangement orequivalents which can be obtained by a person skilled in the art withoutcreative work or undue experimentation. The scope of the claims,therefore, should be accorded the broadest interpretation so as toencompass all such modifications and similar arrangements andequivalents.

What is claimed is:
 1. A photodetector comprising: an n-type structure,an UV transparent p-type structure, and a photon absorbing regionsandwiched between the n-type structure and the UV transparent p-typestructure; an UV transparent p-contact layer formed on the UVtransparent p-type structure, wherein the UV transparent p-contact layeris a strained Mg-doped AlGaN layer with Al-composition in the range of0.7-1; and a p-ohmic contact of a thickness in the range of 0.2-100 nmformed on the p-contact layer, wherein the p-ohmic contact comprises oneor more layer of metal oxide, wherein the metal oxide is a binary oxideselected from NiO_(x), InO_(x), SnO_(x) and RhO_(x), or a ternary oxideof formula CuMO_(x), where metal M is selected from Al, Ga, In, Cr, Sr,Sc, and Y; or is a spinel type oxide of formula ZnM₂O_(x), where metal Mis selected from Rh, Ir, or Co; and when the p-ohmic contact comprisesmore than one layer of metal oxide, each layer of metal oxide isindependently selected from the binary oxide, the ternary oxide, and thespinel type oxide.
 2. The photodetector of claim 1, wherein the p-ohmiccontact comprises one layer of the metal oxide.
 3. The photodetector ofclaim 1, wherein the p-ohmic contact comprises two layers of metaloxide, selected from a NiO_(x) layer facing the UV transparent p-contactlayer and a MoO_(y) layer on the NiO_(x) layer, a NiO_(x) layer facingthe UV transparent p-contact layer and a InO_(y) layer on the NiO_(x)layer, a NiO_(x) layer facing the UV transparent p-contact layer and aRhO_(z) layer on the NiO_(x) layer, a RhO_(x) layer facing the UVtransparent p-contact layer and a MoO_(y) layer on the RhO_(x) layer, aMoO_(x) layer facing the UV transparent p-contact layer and a RhO_(y)layer on the MoO_(x) layer, a InO_(x) layer facing the UV transparentp-contact layer and a NiO_(y) layer on the InO_(x) layer, a InO_(x)layer facing the UV transparent p-contact layer and a MoO_(y) layer onthe InO_(y) layer, or a InO_(x), layer facing the UV transparentp-contact layer and a RhO_(y) layer on the InO_(x) layer, wherein athickness of the metal oxide layer facing the UV transparent p-contactlayer in the two layers of metal oxide is 0.1%-5% of a total thicknessof the two layers of metal oxide.
 4. The photodetector of claim 1,wherein the p-ohmic contact is of a thickness in the range of 1-10 nm.5. The photodetector of claim 1, further comprising an assistantp-contact layer formed on the UV transparent p-contact layer, whereinthe assistant p-contact layer is made of p-type AlGaN with anAl-composition in the range of 0-40% and a thickness in the range of0.2-2 nm.
 6. The photodetector of claim 1, further comprising a metalreflector layer formed on the p-ohmic contact.
 7. The photodetector ofclaim 6, wherein the metal reflector layer is made from a metal selectedfrom Al, Pd, Pt, Os, Rh, Ir, In, Mo, and W and has a thickness in therange of 10-200 nm.
 8. A photodetector comprising: an n-type structure,an UV transparent p-type structure, and a photon absorbing regionsandwiched between the n-type structure and the UV transparent p-typestructure; an UV transparent p-contact layer formed on the UVtransparent p-type structure, wherein the UV transparent p-contact layeris a strained Mg-doped AlGaN layer with Al-composition in the range of0.7-1; and a p-ohmic contact of a thickness in the range of 0.2-100 nmformed on the UV transparent p-contact layer, wherein the p-ohmiccontact is formed by sequentially depositing one or more layers of metalon the UV transparent p-contact layer and oxidizing the one or morelayers of metal to form metal oxide, wherein the metal oxide is a binaryoxide selected from NiO_(x), InO_(x), SnO_(x) and RhO_(x), or a ternaryoxide of formula CuMO_(x), where metal M is selected from Al, Ga, In,Cr, Sr, Sc, and Y; or is a spinel type oxide of formula ZnM₂O_(x), wheremetal M is selected from Rh or Co.
 9. The photodetector of claim 8,wherein oxidizing the one or more layers of metal is conducted byannealing the one or more layers of metal in oxygen-containing ambientat a temperature in the range of 450-850° C. for a period of 1-20minutes.
 10. The photodetector of claim 9, wherein the p-ohmic contactis formed by sequentially depositing two layers of metal on thep-contact layer and oxidizing the two layers of metal, the two layers ofmetal include a first layer formed on the p-contact layer and a secondlayer formed on the first layer and are selected from: a Ni layer on thep-contact layer and a Mo layer on the Ni layer, or an In layer on the Nilayer, or a Rh layer on the Ni layer; a Mo layer on the p-contact layerand a Ni layer on the Mo layer, or an In layer on the Mo layer, or a Rhlayer on the Mo layer; an In layer on the p-contact layer and a Ni layeron the In layer, or a Mo layer on the In, or a Rh layer on the In layer;a Rh layer on the p-contact layer and a Mo layer on the Rh layer, or aNi layer on the Rh, or a Pd layer on the Rh layer; wherein a thicknessof the first layer is in the range of 0.2-2 nm, and a thickness of thesecond layer is in the range of 2-100 nm, optionally 5-20 nm.
 11. Thephotodetector of claim 9, wherein the p-ohmic contact is formed bysequentially depositing three layers of metal on the p-contact layer andoxidizing the three layers of metal, the three layers of metal include afirst layer formed on the p-contact layer, a second layer formed on thefirst layer and a third layer formed on the second layer, and areselected from Ni, Rh, Mo, Pd, Ir, Os and Ru, respectively; wherein athickness of the first layer, the second layer and the third layer arein the range of 0.2-2 nm, 1-20 nm, and 1-20 nm, respectively.
 12. Thephotodetector of claim 8, wherein, during oxidizing the one or morelayers of metal, oxygen penetrates into the UV transparent p-contactlayer and the UV transparent p-type structure.
 13. The photodetector ofclaim 8, wherein the p-ohmic contact is of a thickness in the range of1-10 nm.
 14. The photodetector of claim 8, further comprising a metalreflector layer formed on the p-ohmic contact.
 15. The photodetector ofclaim 8, further comprising a template layer, wherein the n-typestructure is formed on the template layer, and both the template layerand the n-type structure are UV transparent.
 16. The photodetector ofclaim 15, wherein an Al-composition of the template layer is 20-50%higher than an Al-composition of the photon absorbing region.
 17. Thephotodetector of claim 8, wherein the photon absorbing region is made ofintrinsic AlGaN with a thickness of 100-500 nm.